1. Field of the Invention
The present invention relates to a device for treating handling a material web, such as a paper or cardboard web. More particularly, the material web is guided by at least one smooth support surface and is then guided together with a porous support belt to a deflection roll or the like. The material web leaving the support surface is drawn against the porous support belt by a vacuum. The vacuum is provided within a vacuum zone defined by a side of the porous support belt facing away from the material web and an opposing wall of a suction box. The vacuum zone is sealed on its front end in the web travel direction by at least one scaling element that cooperates with the support belt and extends with its rear end to the deflection roll. The suction box wall is provided with a slot-shaped first suction opening in the vicinity of the rear end of the vacuum zone. Air can be aspirated from the vacuum zone through the opening.
2. Discussion of Background Information
In a device known from DE-A1-195 27 289, the suction box wall opposite from the porous support belt is only provided with a suction opening near the rear end, in terms of the web travel direction.
As soon as the material web leaves the support surface, which is embodied, for example, as a drying cylinder, it is held by the vacuum against the back side of the porous support belt, which is embodied, for example, as a drying wire. The prior suction box principle, however, has several disadvantages. Even if the scaling element rests directly against the porous support belt, which is practically never the case, air cannot be prevented from filling the vacuum zone, even in the immediate vicinity of this sealing element. This can be attributed to the porous nature of the support belt, which permits air flow. Thus, a pressure loss occurs after the sealing element. Moreover, the still-moist material web has a tendency to adhere to the smooth support surface. Consequently, a vacuum sufficient to draw the material web to the porous support belt is only available within a certain distance from the sealing element.
Problems also arise when beginning each material web which, as a rule, is carried out in two steps. Thus, upon insertion of the tip or the leading edge of a material web without ropes, because of the adhesion to the smooth support surface as well as the lack of negative pressure behind the uncovered or at best only slightly covered support belt, the tip does not detach from the smooth support surface, As a result, the tip remains adhered to the support surface until it comes to a scraper, which is customarily provided, such that it finally returns to the support belt by means of air pressure. Even when the tip is carried along by the porous support belt, air can still travel through the support belt into the vacuum zone, especially around the sides of the tip, resulting in a significant weakening of the vacuum. Consequently, a vacuum sufficient to draw the material web against the support belt and secure it there is only available near the suction slot provided close to the rear end of the vacuum zone.
Up until now, adhesion of the web to the smooth support surface has always been counteracted b dividing the Suction box and/or the vacuum zone to generate a more intense vacuum, for example, across the width of the tip. Such a box structure is, however, expensive and complex, particularly because appropriate dividers are required. Only when the material web has widened out to its full extent and the porous support belt is covered at least substantially across its entire width, is there sufficient negative pressure in the entire vacuum zone to draw the material web against the support belt and secure it there. However, external air can also easily penetrate through the porous support belt and around the two lateral web edges, i.e., both on the operator end and on the driven end, into the vacuum zone in this operational phase. Such penetration occurs in the region of the sealing element that cooperates with the support belt.
During operation, edge lifting or raising of the material web occurs on the straight path between the smooth support surface and the deflection roll. This lifting can occur both on the operator end and on the driven end where a special tip width region or edge strip region is not usually provided. The edge lifting is irregular and short in duration, so that a kind of web fluttering occurs. The primary cause for this lifting is the weakened vacuum after the sealing element as well as a loss in negative pressure at the two lateral edges of the material web. Air turbulence occurring between the material web and the support surface, contribute to such web lifting, which occurs near the straight course of material web between the smooth support surface and the deflection roll--both on the operator side and on the driven side.